1. Field of the Invention
The present invention pertains, in general, to an insulating film having improved adhesive strength and a printed circuit board having the same. More specifically, the present invention pertains to an insulating film for use in an insulating layer of a multilayer printed circuit board, in which a portion adjacent to CCL (copper clad laminate) is different in composition from a desmear-treated portion adjacent to a plating layer to increase an adhesive strength between the insulating layer and the plating layer upon preparation of the multilayer printed circuit board by the build-up process, and a printed circuit board having the same.
2. Description of the Related Art
In accordance with a recent trend of minaturized and digitalized high-frequency electronic apparatuses, electronic parts and boards have been required to have high integration and high-density properties. Conventionally, there is known a process for manufacturing a multilayer printed circuit board, in which copper foils are laminated in a press on an internal layer circuit board having a circuit formed thereon by using as insulating bonding layers several prepreg sheets each prepared by impregnating glass cloth with an epoxy resin, and semi-curing it. This process has, however, had problems including a high cost incurred by large-scale equipment and a long period of time required for molding under heat and pressure by a laminating press, the formation of a fine pattern made difficult by an increased copper thickness due to through-hole plating on an external layer, and high dielectric constant of the circuit board.
Attention has recently been drawn, as means for solving those problems, to the technique by which organic insulating layers are formed alternately on conductor layers of an internal layer circuit board for making a multilayer printed circuit board of the build-up type. Therefore, via holes are formed and thus a wiring density is increased according to the build-up process. Additionally, the circuit board having high density and made by a thinner sheet has been formed by a laser process. Conventionally, liquid insulation resins and photo-sensitive insulation resins have been used as the insulating layers in the multilayer board of the build-up type. However, because of the formation of an uneven surface on the insulating layers, and the limited selection of the insulating materials for the preparation of the board, it is preferred that a semi-solid dry film type insulating materials are used. In the case of liquid insulation resins, the insulating layer can be easily formed through coating process, however, upon coating on a board, the uneven surface on the insulating layer is generated owing to foaming and also productivity is lowered caused by sectional processing. The photo-sensitive insulation resins can collectively form greater numbers of holes, but suffers from the disadvantages of being expensive, and size limitation of fine via holes (minimum 80 xcexcm). The dry film type insulation resins have the advantages of easily controlling a thickness of the insulating layer, thinning a board, and good process efficiency by simultaneously processing both sides of the layer. In addition, fine holes (50 xcexcm or lower) can be formed because of processing the board with laser. But the insulation resins are disadvantageous in terms of insufficient adhesive strength between an insulating layer and a conductive layer.
Further, as a pretreatment process of copper plating, the desmear process is carried out, whereby the internal layer circuit is smoothly plated with the external layer circuit due to removal of any smear of resin generated upon drilling, and roughness becomes high on the resin surface, thus increasing the adhesive strength of the plating layer versus the insulating layer. In the desmear process, the insulating layer is swelled with oxidizing agents, such as permanganate, bicromate, hydrogen peroxide and the like, thereby removing the filler in the insulating layer and increasing the roughness on the insulating layer. Therefore, upon electrolytic or electroless plating, the plating layer is smeared between the convexoconcave portions formed on the insulating layer, so improving the adhesive strength between the plating layer and the insulating layer.
Recently, U.S. Pat. No. 6,133,377 of Ajinomoto Co. Inc., Japan, discloses a process for increasing fine roughness by thermosetting a phenol epoxy resin including a triazine ring, in preparation of dry film type insulation resins. According to the process of Ajinomoto Co. Inc, the adhesive strength between the insulating layer and the plating layer is improved, without mechanical grinding process or chemical swelling process. However, when the board is practically prepared, chemical swelling process in which the desmear treatment is performed and the filler in the insulating layer is discharged is carried out, thus increasing the adhesive strength between the layers.
With the problems encountered in the prior art in mind, the present invention has an object of providing an insulating film having improved adhesive strength by effectively performing the desmear treatment.
It is another object of the present invention to provide a printed circuit board having the insulating film being improved in an adhesive strength with a plating layer.
In accordance with an aspect of the present invention, there is provided an insulating film for use in an insulating layer of a multilayer printed circuit board, the insulating film comprising: a first coating layer comprising an epoxy resin, a rubber, and a filler and a second coating layer, located on the first layer and comprising an epoxy resin, a rubber, and a filler, wherein the first coating layer having a greater amount of rubber and filler and a smaller amount of epoxy resin than the second coating layer.
In accordance with another aspect of the present invention, there is provided a multilayer printed circuit board having the insulating film.